In a coated optical fiber, an optical fiber drawn from melted glass is coated with a resin for the purpose of protection and reinforcement. However, even when the coating resin is in a stabilized state after curing, a physical minute residual stress or residual strain may be generated in the coating resin due to a stress or strain newly loaded on the optical fiber in a process of rewinding, etc. or at the time when temperature changes. In the case where the distribution of this residual stress/strain in the optical fiber is non-uniform, a transmission loss, called a microbending loss may be generated in some cases.
As a method for preventing propagation of an external force causing such a microbending loss to an optical fiber, there is known a method of enhancing microbending loss resistance by decreasing the Young's modulus of a primary coating layer, which is in contact with the optical fiber, to impart a performance of a cushioning material while the Young's modulus of a secondary coating layer is made high to enhance rigidity.
For example, in Patent Document 1 relating to an optical fiber having the microbending loss resistance, it is disclosed that the Young's modulus of the primary coating layer is controlled to 0.55 MPa or less and the Young's modulus of the secondary coating layer is controlled to 500 to 1,500 MPa.